In the well-known process of electrophotography or xerography, a photoconductive member is charged to a substantially uniform potential so as to sensitize its surface. The charged portion of the photoconductive surface is exposed to a light image of an original document being reproduced. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing toner particles into contact therewith. The toner particles are attracted to the latent image forming a toner powder image on the photoconductive member. The toner powder image is then transferred from the photoconductive member to a copy sheet. Finally, the copy sheet is heated to permanently affix the toner particles thereto in image configuration.
In a practical application, the location of the latent image recorded on the photoconductive belt must be precisely defined in order to have the various processing stations acting thereon optimize copy quality. To this end, it is critical that the lateral alignment of the photoconductive belt be controlled within prescribed tolerances. Only in this manner will the photoconductive belt move through a pre-determined path so that the processing stations disposed thereabout will be located precisely relative to the latent image recorded thereon.
When considering control of the lateral movement of a belt, it is well known that if the belt were constructed and entrained about perfectly cylindrical rollers mounted and secured in an exactly parallel relationship with one another, there would be no lateral movement of the belt. In actual practice, however, this is not feasible. Frequently the belt velocity vector is not normal to the roller axis of rotation, or the roller is tilted relative to the plane defined by the moving belt. Under either of these circumstances, the belt will move laterally relative to the roller until it is in a stable position. In any control system, it is necessary to prevent high local stresses which may result in damage to the highly sensitive photoconductive belt. Active systems, such as servo systems employing steering rollers apply less stress on the belt. However, active systems of this type are generally complex and costly. Passive systems, such as flanged rollers, are less expensive but generally produce high stresses.
Various types of flanged roller systems have hereinbefore been developed to improve the support and tracking of photoconductive belts. For example, the drive roller may have a pair of flanges secured to opposed ends thereof. If the photoconductive belt moves laterally, and engages one of the flanges, it must be capable of sliding laterally with respect to the drive roller to maintain its position. The edge force required to shift the belt laterally greatly exceeds the maximum tolerable edge force. Thus, the belt would start to buckle resulting in failure of the system. Belt edge forces are large because the drive roller has no lateral compliance. Unless the approach angle of the belt, when it contacts the drive roller, is exactly zero, forces large enough to slide the belt with respect to the drive roller are generated. Thus, a system of this type is not always satisfactory for controlling lateral movement of a photoconductive belt in an electrophotographic printing machine.
U.S. Pat. No. 4,221,480 discloses a roller, on which a photoconductive belt is entrained, defining a series of disc-shaped members extending from a central core. The edge of each disc contacts the belt. Each disc has resilient properties and is spaced from adjacent discs by an appreciable distance along the roller. Small deformations of certain discs caused by lateral motion of the belt relative to the roller are counteracted by resilience of the discs, which has an effect of aligning the belt. One practical problem with the arrangement described in the '480 patent is that, under intense use, the discs act as heat sinks with greater effectiveness than the air between the discs, resulting in small temperature differentials between the disc-contacting and the non-contacting portions of the belt. In a xerographic context, these small differences in temperature result in differences in xerographic development performance. Prints made in the presence of these temperature differentials may exhibit stripes of varying image darkness along the direction of motion of the belt.
U.S. Pat. No. 3,070,365 discloses a roller for supporting a xerographic image receptor belt. The roller includes two oppositely-wound helical springs attached to the roller with an adhesive. The action of the two springs in the rotating roller aids in maintaining alignment of the image receptor belt.